The invention relates to a unit comprising a short-arc discharge lamp and a starting antenna, the short-arc discharge lamp including a translucent, gas-tight lamp vessel with an ionizable fill. A a first and a second electrode are arranged in the lamp vessel of which either one of the two electrodes is connected to a current conductor of its own extending to outside the lamp vessel, a starting antenna connected to a further current conductor being arranged near to the lamp vessel.
Such a unit comprising a short-arc discharge lamp and starting antenna is known from U.S. Pat. No. 4,053,809. A short-arc discharge lamp, hereinafter to be denoted lamp, is understood to mean a discharge lamp of which the distance between the electrodes is smaller than half the widest outside diameter of the lamp vessel. The short discharge arc makes a proper bundling of the light generated by the lamp possible. This renders the lamp highly suitable as, for example, a projection lamp or a car headlamp. Short-arc discharge lamps have a fill that adopts a very high pressure of the order of several tens of bars and upwards during the operation of the lamp. The fact that the high pressure decreases only gradually after switch-off renders it difficult to reignite the lamp shortly afterwards. With the known lamp, the lamp vessel has a central portion and neck-shaped end portions on either one of the two sides thereof. Alongside the lamp vessel is extended a metal conductor which is attached encircling one of the end portions at a distance from the central portion. The further current conductor to which the metal conductor is connected is in its turn connected to one of the current conductors of the electrodes. The metal conductor realizes a shortening of the time (reignition time) that is necessary for reigniting the lamp and therefore operates as a starting antenna The reignition time is shorter as a higher reignition voltage is applied to the starting antenna. The permissible reignition voltage on the starting antenna, however, is limited because spark-over from the starting antenna to the lamp vessel occurs when voltages are too high. This causes damage to the lamp vessel which considerably shortens the life of the lamp.
It is an object of the invention to reduce the reignition time possible and to avoid spark-over from the starting antenna to the lamp vessel. According to the invention the starting antenna has a gas-tight antenna container with an ionizable fill and includes a further electrode which is connected to the further current conductor. When the further current conductor produces a reignition voltage, the further electrode causes an ionization to occur of the ionizable fill of the antenna container. The fill of the antenna container has then become conductive, so that this generates an electric field in the lamp vessel similarly to a metal conductor. Surprisingly, however, it has appeared that with the unit according to the invention a considerably higher reignition voltage can be produced on the starting antenna without spark-over from the starting antenna to the lamp vessel. This makes a further reduction of the reignition time possible.
It is noted from U.S. Pat. No. 5,248,918 that an electrodeless HID lamp is known in which, by means of magnetic induction, an electric discharge in the ionizable fill of the lamp vessel is maintained during operation. Due to the lack of electrodes, such lamps in cold state are generally harder to ignite than conventional lamps which do have electrodes. An important cause of this is a shortage of free electrons in the lamp vessel of the electrodeless lamp in that state. To improve the ignition of this electrodeless lamp, a tube containing an ionizable medium is attached to the lamp vessel. When the electrodeless lamp is ignited, a high voltage is offered at a free end of the tube. The measure according to the invention is especially effective when the short-arc discharge lamp is reignited in hot condition. For the hot reignition of short-arc discharge lamps, a lack of free electrons does not play any role.
Furthermore, there is noted that from U.S. Pat. No. 3,828,214 is known a high-pressure sodium lamp of which the lamp vessel is included in an envelope which contains an ionizable fill. Further electrodes are arranged inside the envelope. With this lamp, the distance between the electrodes of the lamp is considerably larger than the diameter of the lamp vessel. When a voltage is applied to the lamp, the fill in the envelope enveloping the lamp vessel is ionized. The fill in the lamp vessel is heated as a result, so that the starting voltage of the high-pressure sodium lamp drops. In this patent application is stated that the ionized plasma in the envelope acts as a conductive body. A conductive body, for example, a conducting strip, in the vicinity of the lamp vessel is used in high-pressure sodium lamps to shorten the distance to be bridged on ignition. First a capacitive discharge arises over a relatively short distance between the conductive strip and a neighboring electrode. After that, the discharge in the lamp vessel is extended to between the electrodes in the lamp vessel.
Experimental examination of the unit according to the invention has shown the inventors that after a reignition voltage is applied to the starting antenna, there is first an initial discharge over a relatively long path along an inside surface of the wall of the lamp vessel. Subsequently, this initial discharge turns into an arc discharge between the electrodes.
In a short-arc discharge lamp the density of the ionizable fill in hot condition is very high. This makes it difficult to sufficiently accelerate free electrons present in the lamp vessel, so as to realize a discharge. Consequently, a rise of the temperature of the lamp vessel when the short-arc discharge lamp is ignited actually leads to an increase of the starting voltage. In an advantageous embodiment of the unit according to the invention, the lamp vessel is for this reason arranged outside the antenna container. As a result, the heating of the lamp vessel as a result of the discharge in the antenna container is substantially avoided.
For counteracting optical losses, the antenna container is preferably made of a translucent material, for example, a ceramic material such as monocrystalline metal oxide, for example, sapphire, polycrystalline metal oxide, for example, translucent gas-tight aluminum oxide (DGA), yttrium aluminum grenate (YAG) or yttrium oxide (YOX), or polycrystalline non-oxidic material such as aluminum nitride (AlN). Glass, for example quartz glass, is also suitable as a translucent material and has the additional advantage that it provides a relatively large freedom of form of the starting antenna.
In a unit according to the invention, the nature and intensity of the radiation generated in the antenna container for achieving a shorter reignition time is not of prime importance. However, for achieving a short ignition time when the lamp is ignited in cold condition, in the absence of ambient light, it is favorable if the starting antenna in an activated condition generates UV radiation, preferably in a wavelength band from 190 to 260 nm. For example, the starting antenna has a fill of mercury and argon.
The further electrode may be included in the antenna container and connected to the further current conductor via a gas-tight lead-in. However, an embodiment in which the further electrode is attached to an outside surface of the antenna container is easier to manufacture. A gas-tight lead-in is then not necessary. In addition, this enhances the options with respect to the materials for the further electrode and with respect to the components of the fill, because the wall of the antenna container in this case avoids any chemical interactions between the further electrode and the fill inside the antenna container.
The reignition voltage produced on the starting antenna is, for example, a high-frequency A.C. voltage, but, on the other hand, may be a possibly recurrent, pulsatory voltage.
In an advantageous embodiment, the unit according to the invention is further characterized by voltage-transforming means in which the current conductors are connected to an input of the voltage-transforming means and in that the further current conductor is connected to an output of their own of the voltage-transforming means. Since the unit includes voltage-transforming means, it may be connected to a power supply which needs to supply only a relatively low voltage both on ignition of the lamp and during nominal operation of the lamp; Therefore, relatively cost-effective components may be used for the power supply. The voltage-transforming means are arranged, for example, as a transformer, for example having a primary winding and a secondary winding around a core of magnetizable material. On the other hand, the transforming means may be arranged as a spiral line transformer.
It is attractive if the voltage-transforming means are formed by a piezoelectric transformer. For a frequency near to its resonance frequency, a transformer of this type produces a considerably higher output voltage than for a frequency that deviates more from the resonance frequency. This is especially advantageous in embodiments in which the transformer and the lamp are connected to the same power supply, because in this manner the voltage on the output of the transformer may be changed without this having an appreciable effect on the voltage on the electrodes of the lamp.
An attractive embodiment of the unit according to the invention is characterized in that the lamp vessel has a relatively wide central portion and on either one of the two sides thereof neck-shaped end portions, with the electrodes being arranged in the central portion of the lamp vessel, the current conductors extending each through a respective end portion, and the antenna container of the starting antenna being a tube which encircles one of the end portions near to the central portion. This double-sided short-arc discharge lamp lends itself fairly easily for manufacture on an industrial scale.
Owing to the short distance between the electrodes, the short-arc discharge lamp is eminently suitable for use in a unit with a reflector, for example, for projection purposes.
Preferably, the unit includes the above-described double-sided short-arc discharge lamp. A practical and compact embodiment of such a unit is characterized in that the reflector is a converging reflector having an optical axis, a light emission window and, opposite this window, a further window with the reflector encircling the central portion of the lamp vessel, the neck-shaped portions of the lamp vessel extending along the optical axis and the end portion encircled by the starting antenna extending outwardly through the further window.
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.